Frequency generator



Sept. 8,1953

Filed Nov. 16, 1951 FIGJ.

O. S. FIELD FREQUENCY GENERATOR s Sheets-Sheet 1 INVENTOR.

by 0.5.Field H i s ATTORNEY Sept. 8, 1953 o. s. FIELD 2,551,734

- I FREQUENCY-GENERATOR Filed Nov. 16, 1951 6 Sheets-Sheet 2 INVENTOR.

By 0. 5. Field Hi 3 ATTORNEY FIG.2.

0. S. FIELD FREQUENCY GENERATOR Sept. 8, 1953 6 Sheets-Sheet 3 Filed Nov. 16, 1951 mohgm mmtbo INVENTOR.

His ATTORNEY 0.5m QM IHOE.

Judi

Sept. 8, 1953 o. s. FIELD FREQUENCY GENERATOR Filed Nov. 16, 1951 6 Sheets-Sheet 4 INVENTOR. O. 5. Fie Id Maw,

His ATTORNEY Sept. 8, 1953 O. S. FIELD FREQUENCY GENERATOR Filed Nov. 16, 1951 6 Sheets-Sheet 5 FREQUENCY- VOLTAGE CONVERTER HG. 6B. amass g3 91 /4?! ROTOR I SHUNT FIGJB. imam:

ROTOR iwm AYWXZMI' iNVENTOR.

His ATTORNEY Sept. 8, 1953 o. s. FIELD 2,651,734

I FREQUENCY GENERATOR Filed Nov. 16, 1951 6 Sheets-Sheet s INVEN TOR.

M/ZM

His ATTORNEY FIG-.8.

Patented Sept. 8, 1953 FREQUENCY GENERATOR Oscar S. Field, Rochester, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Application November 16, 1951, Serial No. 256,634

7 Claims.

This invention relates to frequency generators, and it more particularly pertains to a generator having a toothed rotor and a toothed stator cooperating to furnish an output frequency varying directly with the speed of the rotor while the generated voltage, when applied to a substantially constant load, remains substantially constant over a wide range of speeds.

In railway practice it is desirable to obtain accurate indications as to the speed of a locomotive at a point on the locomotive remote from the wheel axle, and difficulties, such as vibration and the like, make it impractical to use a conventional speedometer chain connection from an axle to the speed indicator. By the present invention the speed indicator can be driven electrically with a high degree of accuracy. The generator according to the present invention also has utility in providing speed indication for train control purposes, such, for example, as in the train control system disclosed in the application of J. E. Freehafer, Ser. No. 256,628 filed of even date with the present application.

Generally speaking, and without attempting to define the scope of the present invention, the present invention comprises a generator for coupling directly to a locomotive axle, the generator having a frequency output variable directly with the speed of the locomotive, and maintaining a substantially constant voltage over a wide range of speeds when a substantially constant load is applied to the generator.

The generator structure comprises a magnetic rotor and stator, a magnetic shunt, and a permanent magnet for supplying the magnetic field. The rotor and stator and shunt have teeth cooperating to alternately form a magnetic generating circuit and a magnetic shunt circuit as the rotor is driven by the locomotive axle. By reason of use of the relatively large number of teeth, and the shunt magnetic circuit, it is possible to generate suflicient voltage for satisfactory operation of a speed indicator when the locomotive is moving at a very low speed, and as the speed of the locomotive increases, the increase in frequency is ellective to offer a greater impedance in the load circuit, and thus maintain the output "voltage under constant load conditions substantially constant.

An object of the present invention is to generate a frequency indicative of speed by the turning of a toothed rotor coupled to the axle of the locomotive, the rotor having two sets of teeth, one set of which cooperates with a particular set of stator teeth for varying the field cutting the winding of the generator, and the other set of rot r t t ati g w th a et of teeth of the magnetic shunt for shunting out the generating magnetic field at times when the coop crating rotor and stator teeth are out of register opposite each other.

Another object of the present invention is to use a relatively large number of cooperating teeth on the rotor and stator as an eificient means for generating at low axle speeds.

Another object of the present invention is to use the output of the frequency generator to control a speed indicator or the like at a point on the locomotive remote from the axle. I

Other objects, purposes, and characteristic features of the present invention will be in part obvious from the accompanying drawings, and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference is made to the accompanying drawings, in which corresponding parts are identified by similar reference characters throughout the different views; and in which:

F g. 1 is an elevational view, shown partly in cross-section, of the frequency generator and its housing as it is attached to the hub of an axle of a locomotive;

Fig. 2 is an elevational sectional View of the frequency generator shown in Fig. 1

Fig. 3 is a fragmentary sectional view of the cooperating teeth of the frequency generator shown in Fig. 2, taken along the section line 3-3 of Fig. 2;

Fig. 4 is an exploded elevational view, shown partly in cross-section, and with certain parts broken away, illustrating the cooperation of the respective toothed elements of the frequency generator shown in Fig. 1

Fig. 5 is a view in perspective, of th cooperating rotor, stator, and magnetic shunt toothed elements of the generator shown in Fig. 1, illustrating particularly the cooperating relationship of the teeth of the respective elements;

Figs. 6A and 6B illustrate diagrammatically how the generating fiux is set up in the generating magnetic structure when the teeth of the cooperating elements are in alignment as illustrated in Fig. 6B; v

Figs. 7A and 7B illustrate diagrammatically how the generating flux is shunted by the magnetic shunt when the teeth of the respective elements cooperate as is illustrated in Fig. B;

Fig. 8 is an elevational sectional view of a modified type of frequency generator wherein the rotor is a permanent magnet;

Fig, 9 is a fragmentary enlarged perspective view, particularly showing the arrangement of the teeth in the generator shown in Fig. 8; and,

Fig. 10 is a plan view of a locking disc used in the generator shown in Fig. 8.

With reference to Fig. l, the general organization comprises a cylindrical case C suitably Secured by bolts Hi to a conventional locomotive journal box JB, a generator G suitably secured within the case C by bolts II, and a cover IQ for enclosing the generator G within the case C when fastened to the case C by bolts l3.

The generator G, according to Figs. 1 and comprises a supporting sleeve l4, a drum-shaped stator S, a stator winding l5, a permanent magnet PM in the form of a disc, a magnetic shun SH in the form of a disc and a rotor R.

A rotor shaft |6 extends through the center of the supporting sleeve l4 and is iournaled by conventional radial ball bearings l1 and H3 near the respective ends of the shaft It. The inner races of the ball bearings l1 and I8 bear against the respective shoulders l9 and 20 Of the shaft IS, the outer race of the bearing I! being secured in position against the shoulder 2| within the supporting sleeve M by a ring 22 fitted into a suitable slot formed in the sleeve l4. Between the two bearings, a rubber bushing in a sheet metal retainer is retained against the shoulder 24 within the supporting sleeve 14 by a suitable retainer ring 25 fitted into a slot formed in the supporting sleeve M. This bushing blocks oil and grease from working through to the rotor R.

The right-hand end of the shaft as viewed in Fig. 2 is adapted to fastened in a coupling 28. This coupling serves as a coupling linkage for connection to a shaft '21 (see Fig. 1) which is driven directly by the rotation of the locomotive axle. Connection of the shaft It to the coupling 26 is made by the right-hand end of the shaft I6 being inserted in a hole 28 in the left-hand end of the coupling 25. The right-hand end the shaft iii has a key way, and the coupling of the shaft I6 to the coupling 26 is by a washer 29 having a suitable detent engaging the key way, and having four protruding ears to which lock in engagement with cooperating recesses 3| in the coupling 26 so as to cause the shaft l3 to be driven by rotation of the coupling 26, and permit end play within the joint between the shaft l6 and the coupling 26. The right-hand end of the coupling 26 has a longitudinal slot 32 for receiving the tongue 33 of the shaft 21 (see Fig. 1) which is in turn driven by the rotation of the locomotive axle, comparable to the connecting shaft in the patent to H. B. Taylor, No. 2,457,748, dated December 28, 1948. Such tongue 33 is connected to the coupling 26 by a suitable pin or rivet 34 (see Fig. 1) extending through the holes 35 (see Fig. 2) of the coupling 26.

The drum-shaped stator S is constructed of magnetic material, and it is assembled over the supporting sleeve M and against the shoulder 36 of the sleeve M to which it is suitably secured, as by welding, concentric with the sleeve |4.

Teeth 31 are formed about the inner periphery of the stator S, at a point adjoining the lefthand edge of the stator drum as is illustrated in Fig. 2 for cooperating with a like number of teeth on the rotor R. Although the number and shape of the teeth may vary according to the requirements of practice, I28 substantially square teeth have been found to give satisfactory operation.

The stator winding I5 is receivable over the outer periphery of the sleeve I4 and within the stator S. It will be noted from the structure that has been described that a generating magnetic structure is set up having a magnetic circuit extending around three sides of the winding, namely across the outer circumference and righ hand side of the winding I5 as viewed in F 2, and across the inner circumference of the winding through the sleeve 4.

The magnetic shunt disc SI-I has teeth 38 about its periphery conforming to the type and number of teeth used on the stator S. The magnetic shunt SH has a nonmagnetic disc plate 3?; S tably secured thereto as by rivets 4t, and this disc 39 carries a nonmagnetic locking block 4| which is secured thereto by a rivet 42. The locking block 4| has several teeth on opposite edges for cooperation in mesh with the teeth 31 and 38 of the stator S and the magnetic shunt SH respectively. These teeth of the locking block are formed directly opposite each other on opposite edges of the block so that when they are in mesh with the teeth 33 and ti of the stator S and the shunt SH respectively, the teeth of the stator S and the shunt SH are positioned in correspondence opposite each other. The magnetic shunt disc SH is mounted on the supporting sleeve |4 against the shoulder 43, in which position it is held by a nut 44 which is threaded onto the left-hand end of the sleeve l4 as viewed in Fig. 2.

The circular permanent magnet PM is secured to the left-hand side of the magnetic shunt SH as viewed in Fig. 2 by screws 45, and it is ma netized with the left-hand and right-hand faces of the magnet, as viewed in Fig. 2, forming opposite poles. For most desirable operating characteristics, the degree of magnetization of the permanent magnet is such as not to saturate either the stator S or the shunt SH.

The rotor R is of magnetic material and is shaped in the form of a drum 46, open at the right-hand end, which is suitably secured, as by silver solder, onto a nonmagnetic hub 41. The hub 41 is keyed on the shaft Is by the key 48, and the hub 41 is retained on the shaft l6 bearing against a spacer 49 by a nut 5!) and loci: washer 5|, the nut 5|] being threaded onto the left-hand end of the shaft l6 and being locked against rotation by a cotter key 52, or look wiring, or the like. The spacer 49 is of such a thickness as to hold to a minimum clearance between adjoining inner and outer faces of the rotor R and the permanent magnet PM respectively. It will be noted that the permanent magnet PM is of considerably smaller diameter than the inner diameter of the rotor drum 46 so that the magnetic circuit between these elements will have its point of lowest reluctance through the above described adjoining surfaces.

Teeth 53 and 54 are formed about the respec tive outer and inner peripheries of the drum 46 of the rotor R, these teeth being comparable in character and in the number of teeth to cooperating teeth 31 and 38 of the respective stator S and the magnetic shunt SH. The teeth 53 and 54 of the outer and inner peripheries of the rotor respectively are, however, out of correspondence with each other so that the teeth 54 cooperating with the shunt SH, for example, are directly opposite radially the spaces between the teeth of the shunt SH when the teeth 53 are directly opposite radially the teeth 31 of the stator S. This condition is clearly illustrated in Fig. 3. The portion of the rotor drum 46 carrying the teeth is disposed between the teeth 31 and 38 of the stator S and the shunt SH respectively so that there is a minimum clearance between the coopcrating sets of teeth, but yet permitting free rotation of the rotor R. Because of the teeth 53 and 54 of the rotor being staggered relative to each other, the rotor teeth 53 and 54 are in correspondence alternately with the teeth 31 of the stator S and the teeth 38 of the shunt SH respectively as the rotor R is rotated.

The electrical connections to the winding l are brought out to respective terminal posts 55 which are suitably secured to a terminal block 56 of insulating material, which in turn is secured by a screw 51 to a supporting bracket 58, the bracket 58 being secured to the outer periphery of the stator S.

The generator G is rubber mounted within the case 0, there being a rubber bushing 59 retained between shoulders 60 and BI of the outer periphery of the sleeve [4 of the generator G. The bushing 59 is fitted within a journal formed in the case C. Thus upon insertion of the generator carrying the bushing 59 within the case C, the bushing 59 extends within the journal in the right-hand side of the case 0 as viewed in Fig. 1 until it strikes the inner shoulder 62 of the case C.

The generator G is secured within the case C by a mounting plate 63 which is fitted freely over the right-hand end of the sleeve l4 prior to the insertion of the rubber bushing 59- onto the sleeve M. Thus after insertion of the generator G within the case 0, it is secured in operating position by the tightening of bolts I l to draw the mounting plate 63 against the right-hand side of the case C as viewed in Fig. 1. This tightening operation compresses the rubber bushing 59 and thus completes the rubber mounting of the generator G within the case C in a position in alignment with the end of the locomotive axle. A rubber washer 64 is used between the starter S and the mounting plate 63 to further seal the inside of the case against grease, dust, and the like leaking through into the housing from the journal box of the locomotive.

Provision is made for wiring connections to the generator G by the threaded opening 65 at the top of the case 0 as viewed in Fig. 1 for receiving a conventional conduit fitting. Thus the wiring connections from the terminal posts 55 of the generator G can be readily fed through the conduit to respective remote points on the locomotive as required in practice. The cover of the case C is tightly sealed by use of a gasket 86.

It is to be understood that the generator G can be used in practically any train control or indicating system where a form of speed indication is required. For example, the generator G is illustrated in Fig. 6A as driving a dial speed indicator 10 through a suitable frequency-voltage converter. The conversion of frequency to voltage for driving the speed indicator 10 can be accomplished in any suitable manner as long as the frequency and voltage to which it is converted are respectively in direct proportion. A suitable converter for this proposal is disclosed, for example, in the January 1945 issue of "Electronics magazine, pages 264 and 268, to which reference is to be made for detailed consideration relative to the construction of the frequencyvoltage converter. The speed indicator 10 can be a suitable voltmeter having a dial marked with uniformly spaced divisions in miles per hour.

In describing the mode of operation of the generator G in governing the speed indicator I0,

ence opposite each other.

reference is made to Figs. 6 and 7, which illustrate respective generating and magnetic shunt conditions that are alternatively set up as the rotor R progresses from tooth to tooth in its rotation relative to the stator S and the shunt SH.

Fig. 6A shows a generating condition which s set up when the flux from the permanent magnet has a magnetic circuit of relatively low reluctance because the teeth 3'! and 53 of the stator S and the rotor R are respectively in correspond- Thus the low reluctance magnetic field for the permanent magnet under this condition extends from the pole at the left-hand surface of the permanent magnet PM, through the drum of the rotor R, teeth 53 and 31 of the rotor drum and the stator S respectively, around the winding l5 and through the supporting sleeve I 4, to the right-hand face of the permanent magnet as viewed in Fig. 6A. Upon the change in flux of this field, a generated voltage in the winding [5 is applied through the frequency-voltage converter to the speed indicator 19.

As the rotor R progresses in rotation so that the above mentioned teeth 53 and 31 of the rotor R and the stator S respectively are out of correspondence as is illustrated in Fig. 7B, the air gap between the out of correspondence teeth has become such as to make the generating magnetic circuit of higher reluctance; while teeth .54 and 33 of the rotor R and the shunt SH respectively are in correspondence to form a magnetic shunt circuit of relatively low reluctance as illustrated in Fig. 7A, so as to effectively rob the generating magnetic circuit of its flux so as to accentuate the change in the generating magnetic flux and thereby generate a voltage in the generator winding l5.

It will thus be seen from the above description, and upon comparing Figs. 6 and '7, that the relatively low reluctance generating and shuntin magnetic circuits are set up alternately as the rotor R progresses from tooth to tooth in its rotation, and by using a relatively large number of teeth, the generator has a suflicient output to actuate the speed indicator 10 when the locomotive is operating at a speed as low as two miles per hour.

A second embodiment of the present invention is illustrated in Figs. 8, 9, and 10, wherein the rotor is a permanent magnet PMI. For convenience in facilitating the description, the same stator and shaft assembly has been used in the embodiment shown in Fig. 8 as has been fully described relative to the embodiment of Fig. 2, and corresponding parts of the two forms of the invention are identified by corresponding reference characters.

The permanent magnet rotor PMI has substantially square teeth I5 formed about its periphery of a character and number to correspond with the teeth of the stator S, except that the teeth 15 of the permanent magnet PM are out deep enough in the periphery of the permanent magnet so that their inside ends efiectively form radial teeth which can cooperate with a magnetic shunting disc SH! for purposes substantially comparable to the purposes that have been described relative to the use of the shunting disc SH of Fig. 2.

The permanent magnet rotor PMI is carried on the shaft 16 of Fig. 8 by a suitable hub 16, which is preferablyof nonmagnetic material such as brass, the hub 16 being keyed on the shaft l6 and retained on the shaft by the nut 50 and lock washer The permanent magnet PMI is secured on the hub 16 between a shoulder 11 and a nut 18 which is threaded onto the hub 16.

The shunting disc SH! is of magnetic material such as silicon steel, and is cup-shaped so as to have a rim at its periphery in which radial teeth 19 are cut, these teeth being of a character to cooperate with the oppositely disposed ends of the teeth 15 about the periphery of the permanent magnet PMI.

The magnetic shunt disc SHI of Fig. 8 is secured on the sleeve M by a nut 44 in a manner corresponding to that which has been described for securing the shunt SI-I on the sleeve 14 in the embodiment disclosed in Fig. 2.

The radial alignment of the teeth 19 of the shunt SHI of Fig. 8 relative to the teeth 31 of the stator S is such that the respective teeth are out of correspondence with each other. The reason for this will be readily apparent with reference to Fig. 9, wherein it is illustrated that in order for the teeth of the shunt SI-Il and the stator S to be in correspondence with the teeth 15 of the permanent magnet alternately, they must be out of correspondence with each other. This out of correspondence condition is maintained by a locking disc 80 (see Fig. of nonmagnetic material such as brass, which is secured by rivets 81 to the back side of the cup-shaped shunt SH! as is shown in Fig. 8. The locking disc 80 has several teeth at spaced points about its periphery as illustrated in Fig. 10 which mesh with the teeth 31 of the stator S so as to maintain the teeth 31 of the stator S out of correspondence with the teeth 19 of the magnetic shunt SHI.

Although the principle of operation of the generator according to Fig. 8 is somewhat the same as that which has been described for the generator according to Fig. 2, the permanent magnet rotor PMI of Fig. 8 is magnetized different from the permanent magnet PM of Fig. 2 in that it is magnetized with opposite poles at its periphery and center respectively. Thus the permanent magnet PMI becomes a series leg of the magnetic circuit extending through the stator S, including its teeth 31, around the winding [5, and through the sleeve [4. The construction of the cup-shaped magnetic shunt is such that points of minimum air gap between the magnetic shunt SHI and the permanent magnet PMI are at the periphery of the shunt and at its hub respectively, so that the shunt effectively bridges the respective poles of the permanent magnet PMI when its teeth cooperate with the ends of the teeth of the permanent magnet PMI in forming a magnetic shunt circuit of low reluctance.

It is believed that it should be readily apparent from the description as set forth, that the mode of operation of the generator according to Fig. 8 follows the general principle of operation as is illustrated in Figs. 6 and 7 and as has been fully described in that the generating magnetic circuit and the shunt magnetic circuit are alternately effective as the shaft I6 is rotated by the locomotive axle so that the permanent magnet PMI progresses in rotation to form correspondence alternately with the stator S and the shunt SHI. The shunt SHI is obviously effective to rob flux from the generating magnetic circuit when its teeth 19 are in correspondence opposite the teeth 15 of the permanent magnet PMI thus completing a low reluctance magnetic circuit between the opposite poles of the permanent magnet PMI, while the teeth 31 of the stator S are out of correspondence with the teeth 15 of the permanent magnet PM I.

Having thus described two specific embodiments of the present invention, it is desired to be understood that these forms have been described more particularly from the point of view of disclosure of the mode of operation and principles involved rather than to limit the scope of the present invention to the specific forms shown, and it is to be further understood that various alterations, adaptations, and modifications may be applied to the specific forms shown in accordance with the requirements of practice except as limited by the appending claims.

What I claim is:

1. A frequency generator comprising in combination, a rotor of magnetic material having teeth uniformly and continuously spaced about its periphery, a stator having a generating magnetic structure and a generating winding, said stator having continuous uniformly spaced teeth disposed opposite the respective teeth of the outer periphery of said rotor to magnetically cooperate therewith, and a fixed magnetic shunting disc having continuous uniformly spaced teeth about its periphery cooperating in correspondence with teeth of said rotor to shunt flux away from said generating magnetic structure at times when the teeth of said rotor and said generating magnetic structure are out of correspondence with each other.

2. A frequency generator comprising, a. toothed stator, said stator having continuous uniformly spaced teeth disposed about its inner periphery and said stator having a generating winding in which a voltage is induced by changes in a magnetic circuit including said teeth, a magnetic shunt disc disposed within said stator, said magnetic shunt disc having continuous uniformly spaced teeth at its periphery, and a magnetic rotor having continuous uniformly spaced teeth about its periphery cooperating with the teeth of said stator and said shunting disc to alternately make up magnetic generating and shunting magnetic fields of minimum reluctance respectively.

3. A frequency generator comprising in combination, a toothed stator, said stator havin continuous uniformly spaced teeth disposed about its inner periphery and said stator having a generating winding in which voltage is induced by changes in the flux of the magnetic circuit including said teeth, a magnetic shunting disc having continuous uniformly spaced teeth at its periphery, said magnetic shunting disc being disposed within said stator, and a magnetic rotor having continuous uniformly spaced teeth about its periphery cooperating with the teeth of said stator and the teeth of said shunting disc respectively.

4. A frequency generator comprising a rotor having first and second sets of continuous uniformly spaced teeth about it periphery, a stator having a winding and a generating magnetic structure with continuous uniformly spaced teeth about its inner periphery cooperating opposite said first set of teeth of said rotor, a permanent magnet providing flux for a magnetic circuit through the teeth of said rotor and said stator, and a shunting magnetic structure having continuous uniformly spaced teeth about its periphery cooperating with said second set of teeth of said rotor to shunt the flux of said permanent magnet away from said Winding, said generating magnetic structure and said shunting magnetic structure having their teeth cooperating with said rotor alternately in circuits of lowest reluctance whereby a maximum change in the magnetic flux of the generating structure is obtained.

5. A frequency generator comprising, a rotor drum of magnetic material having continuous uniformly spaced teeth on both the outer and inner periphery of the drum, a stator having a generating magnetic structure and a winding,

said stator having continuous uniformly spaced teeth spaced opposite the respective teeth of the outer periphery of said rotor to magnetically cooperate therewith, a permanent magnet fixed with respect to rotation and disposed within the drum of said rotor, a magnetic shunt adjoining said permanent magnet having continuous uniformly spaced teeth about its periphery spaced to cooperate magnetically with the teeth of the inner periphery of said rotor.

6. A frequency generator of the character described comprising in combination, a rotor drum of magnetic material having respective sets of continuous uniformly spaced teeth on both the inner and outer periphery of the drum, a stator having a winding and a generating magnetic structure with continuous uniformly spaced teeth cooperatively spaced opposite one of said sets of teeth of said rotor, a permanent magnet providing flux for a magnetic circuit through the teeth of said rotor and said stator, and a shunting magnetic structure having continuous uniformly spaced teeth cooperating with the other set of teeth of said rotor to shunt the flux of said permanent magnet away from said winding, said generating magnetic structure and said shunting magnetic structure having their teeth cooperating with said rotor in respective magnetic circuits of lowest reluctance alternately, whereby a maximum change in the magnetic flux of the generating structure is obtained upon rotation of the rotor from tooth to tooth.

'7. A frequency generator comprising in combination, a toothed stator, said stator having continuous uniformly spaced teeth disposed about its inner periphery and said stator having a generatin winding in which a voltage is induced by changes in the flux of a magnetic circuit including said teeth, a permanent magnet rotor disposed within said stator and having continuous uniformly spaced teeth about its periphery corresponding to and disposed opposite said teeth of the stator, a. fixed shunting disc disposed within said stator having continuous uniformly spaced radial teeth disposed about its periphery corresponding to and opposite the ends of said teeth of said rotor, the teeth of said shunting disc being out of correspondence with the teeth of said rotor when said teeth of said stator are in correspondence with the teeth of said rotor, whereby there is maximum flux change in said magnetic circuit upon rotation of the rotor from tooth to tooth.

OSCAR S. FIELD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 857,394 Hallock June 18, 1907 1,338,324 Neuland Apr. 27, 1920 2,500,730 Yonkers T Mar. 14, 1950 

